Ichnofauna from the Silurian-Devonian beds of the Parnaiba Basin at Poti River Canyon (Piaui State, NE Brazil)

摘要

This paper characterizes the ichnology and sedimentology of the Lower Paleozoic Parnaiba Basin beds exposed in the Poti River Canyon (Piaui State, NE Brazil) and discusses the paleoenvironmental significance of the ichnofauna. The sedimentary succession represents the Silurian-Devonian infill of the Parnaiba Basin and comprises mouth bar deposits (Ipu Formation) and shoreface deposits (Tiangu ' a and Pimenteira formations). The Silurian ichnofauna is represented by Heimdallia isp. in the topmost beds of the Ipu Formation, and Musculopodus sedentarius, Didymaulichnus lyelli, Didymaulyponomos rowei, Diplocraterion isp., Heimdallia isp., Lockeia siliquaria, Nereites irregularis, Palaeophycus tubularis, and Thalassinoides cf. horizontalis in the Tiangu ' a Formation. The Devonian ichnofauna occurs in the Pimenteira Formation and is represented by Beaconites antarcticus, Bifungites munizi, Bifungites crucifomis, Nereites isp., and Rhizocorallium commune. Horizontal burrows and furrows occupying shallow and middle tiers dominate the ichnofauna, suggesting proximal expressions of the Cruziana ichnofacies. The predominance of sandy, unbioturbated, storm-generated beds in the shoreface deposits indicates a high frequency of storm surges in shallower areas of the shelf, which often inhibited or frequently erased most of the post-event substrate colonization record. Thus, the trace fossil in the analyzed succession represents substrate colonization below the low-tide base and above the storm wave base in shoreface settings. The presence of Musculopodus sedentarius in this assemblage expands the geographic and temporal record of the ichnogenus. The similar composition of the trace fossil assemblage with other Lower Paleozoic ichnofaunas reinforces ichnostratigraphic correlations represented in coeval Gondwanan units.